China has placed runways and radar facilities on new islets in the South China Sea, built by piling huge amounts of sand onto reefs.

The construction is straining already taut geopolitical tensions.

The speed and scale of China’s
island-building spree in the South China Sea last year alarmed other
countries with interests in the region. After announcing
in June that the process of building seven new islands by moving
sediment from the seafloor to reefs was almost done, China has focused
its efforts on building ports, three airstrips, radar facilities
and other military buildings on the islands. The installations bolster
China’s foothold in the Spratly Islands, a disputed scattering of reefs
and islands in the South China Sea more than 500 miles from the Chinese
mainland. China’s
activity in the Spratlys is a major point of contention between China
and the United States, and has prompted the White House to send Navy destroyers to patrol near the islands twice in recent months.

Sources: C.I.A., NASA, China Maritime Safety Administration

The new islands allow China to harness a portion of the sea for its own use that had been relatively out of reach. Although there are significant fisheries and possible large oil and gas reserves in the South China Sea, China’s efforts serve more to fortify its territorial claims than to help it extract natural resources, said Mira Rapp-Hooper, formerly the director of the Asia Maritime Transparency Initiative at the Center for Strategic and International Studies, a Washington research group. Though they are too small to support large military units, the islands will also enable sustained air and sea patrols, strengthening China’s influence in the area.

Image by DigitalGlobe, via CSIS Asia Maritime Transparency Initiative

Several reefs have been destroyed outright to
serve as a foundation for the new islands, and the process also causes
extensive damage to the surrounding marine ecosystem. Frank
Muller-Karger, professor of biological oceanography at the University of
South Florida, said sediment “can wash back into the sea, forming
plumes that can smother marine life and could be laced with heavy
metals, oil and other chemicals from the ships and shore facilities
being built.” Such plumes threaten the biologically diverse reefs
throughout the Spratlys, which Dr. Muller-Karger said may have trouble
surviving in sediment-laden water.

Although China was a relative latecomer
to construction in the Spratly archipelago, its island building is much
more extensive than similar efforts by other countries in the area. The
recent activity has unsettled the United States, which has about $1.2 trillion in bilateral trade go through the South China Sea every year.Washington does not recognize China’s ownership of the islands, and in February President Obama reiterated
the government's position that “the United States will continue to fly,
sail and operate wherever international law allows.” To reinforce the
message, the United States Navy sent missile destroyers in October and January within 12 nautical miles of the islands, the conventional limit for territorial waters. According to statements from David Shear,
the top Pentagon official in charge of Asia and the Pacific, the last
time before October that the United States had sent ships or aircraft
that close to the islands was in 2012.

China has built airstrips, ports, radar
facilities, solar arrays, lighthouses and support buildings on the
islands. The airstrips and ports lengthen the reach of Chinese ships and
planes, while the radar facilities allow the country to keep a closer
eye on what is happening nearby. Imagery from January compiled for a recent report by the C.S.I.S. suggests that
China may be constructing a longer-range high-frequency radar
installation on Cuarteron Reef that would help the country monitor air
and ship traffic in the south, farther from the Chinese mainland.

Fiery Cross Reef is one of China’s most
strategically important new islands, with an airstrip that is long
enough to allow the country to land any plane, from fighter jets to
large transport aircraft.

Two additional airstrips on Mischief Reef and Subi
Reef that China has been building since mid-2015 are nearing
completion, bringing China’s total to three airstrips in the region.

Subi reef (Chu-Pi Chiao) with the GeoGarage plateform (NGA chart)

Though China’s airstrips expand the country’s
ability to operate in the South China Sea, they are not the first in the
region — every other country that occupies the Spratlys already
operates an airstrip as well.

Islands and reefs that have undergone recent construction are shown with a white ring.

Colored rings show whether the feature is occupied by China, the Philippines, Malaysia, Vietnam or Taiwan.

Sources: C.I.A., NASA, China Maritime Safety Administration

Vietnam, Malaysia, the Philippines and Taiwan have
also expanded islands in the Spratlys, but at a much smaller scale than
China’s efforts.

China’s reefs hosted smaller structures for years
before the current surge in construction.

By preserving these initially
isolated buildings, China can claim that it is merely expanding existing facilities, similar to what other countries have done elsewhere in the region.